US 2005/0162806 A1 describes a MEMS device for operation at radio frequency (RF). The MEMS device comprises a first electrode and a deformable second electrode. The second electrode is plastically deformable via exposure to thermal energy. This way, an electrical characteristic of the MEMS device can be tuned. The deformable second electrode is also elastically deformable for tuning the electrical characteristic.
During the production of MEMS devices, stresses in the suspended second electrode can occur, which result in an usually undesired plastic deformation of the second electrode. One reason for such undesired stress and plastic deformation is, among others, a difference in thermal expansion coefficients of the MEMS device and of a substrate of the MEMS device. The stress is then caused by high-temperature steps during the fabrication or packaging process. Stress can also be exerted by layers of different materials, which are in contact with the MEMS device and which have different levels of stress after deposition. Stress has also been observed to be caused by capillary forces that are generated during the drying of the MEMS device after a wet processing step during fabrication. Also, stress can be caused by a flow of a liquid or gas along the second electrode.
During operation of MEMS devices, stress can occur as a result of high actuation forces or contact forces on the device, thermal cycling in the presence of different thermal extension coefficients of the MEMS device and its substrate, or as a result of creep. Creep is known as a tendency of materials to plastically deform for relieving stress that is applied over a long period of time at levels that are below the yield stress level. The yield stress level is the stress level, at which a material begins to plastically deform. Plastic deformation is a non-reversible change of shape in response to an applied force.